autotest: provide a custom location to set as sim home

this helps to set a custom home location on sim, if the location is
outside of the locations in Tools/autotest/locations.txt

Usage:

sim_vehicle.sh -l 37.872991,-122.302348,20,260

The location needs to be in the same format as the locations defined in
locations.txt inside autotest

Tools/autotest/sim_vehicle.sh

@@ -39,6 +39,7 @@ Options:
     -A               pass arguments to antenna tracker
     -t               set antenna tracker start location
     -L               select start location from Tools/autotest/locations.txt
+    -l               set the custom start location from -L
     -c               do a make clean before building
     -N               don't rebuild before starting ardupilot
     -w               wipe EEPROM and reload parameters
@@ -66,7 +67,7 @@ EOF
 
 
 # parse options. Thanks to http://wiki.bash-hackers.org/howto/getopts_tutorial
-while getopts ":I:VgGcj:TA:t:L:v:hwf:RNHeMS:" opt; do
+while getopts ":I:VgGcj:TA:t:L:l:v:hwf:RNHeMS:" opt; do
   case $opt in
     v)
       VEHICLE=$OPTARG
@@ -106,6 +107,9 @@ while getopts ":I:VgGcj:TA:t:L:v:hwf:RNHeMS:" opt; do
     L)
       LOCATION="$OPTARG"
       ;;
+    l)
+      CUSTOM_LOCATION="$OPTARG"
+      ;;
     f)
       FRAME="$OPTARG"
       ;;
@@ -268,7 +272,13 @@ popd
 fi
 
 # get the location information
+if [ -z $CUSTOM_LOCATION ]; then
     SIMHOME=$(cat $autotest/locations.txt | grep -i "^$LOCATION=" | cut -d= -f2)
+else
+    SIMHOME=$CUSTOM_LOCATION
+    LOCATION="Custom_Location"
+fi
+
 [ -z "$SIMHOME" ] && {
     echo "Unknown location $LOCATION"
     usage